Fiber based lasers and amplifiers can act as “brightness converters” that convert high power, low brightness pump light to coherent, higher brightness output light. Obtaining higher output powers—key for fiber based devices to become even more competitive with conventional gas and solid stated lasers—typically means delivering higher pump powers to the doped core (typically rare earth doped) of the active fiber of the fiber laser or amplifier. Higher power pump diodes or diode modules typically have a multimode (MM) output that lacks sufficient beam quality to readily directly couple to the small area core of a typical optical fiber. Single mode (SM) pump diodes have better beam quality and can directly couple, but typically have too low a power output.
A double clad (DC) fiber includes a larger inner cladding (“pump cladding”) surrounding the smaller, active core and a second cladding about the inner cladding. MM pump diodes can couple to the larger area of the inner cladding/core and the pump light is absorbed by the active material in the core as the light propagates within the inner cladding. Pump light absorption (typically measured in dB/meter) is significantly reduced compared to core pumping, and so the length of the gain fiber is increased to provide for a total required absorption. For many devices, such as certain continuous wave (CW) devices, which may not involve as high power intensities as pulse devices, the increased length can be a small price to pay for the increased output power, and the DC fiber represented a key advance.
Increased power output (e.g., one or more of average power, peak pulse power, or pulse energy) of pulsed lasers and amplifiers is also of keen interest. However, such devices are more complex, and obtaining increased power output more problematic, especially as the pulse duration decreases, as the peak power can become quite high and the attendant high power intensity can trigger nonlinearities can limit the output power from an amplifying optical device, such as a fiber laser or fiber amplifier. Nonlinear effects can also limit the length of fiber within lasing cavities, making low repetition rate mode locked lasers (e.g., lower than 20 MHz) difficult to design. Pulsed devices, as well as CW amplifying optical devices, could benefit from improvements.